Technologies for engine control apparatuses for automatically stopping an engine of a vehicle during deceleration before the vehicle completely stops in order to enhance fuel economy are available. In such technologies, the slope of a road surface on which the vehicle is travelling is estimated to determine whether it is possible or not to automatically stop the engine in order to prevent the vehicle from rolling down a hill.
One method for determining an estimated value of the slope of the road surface on which the vehicle is traveling is disclosed in Japanese Unexamined Patent Application Publication No. 60-70307. In the disclosed method, a forward/backward acceleration estimated based on a wheel speed of the vehicle, which is obtained from a wheel speed sensor, is subtracted from a forward/backward acceleration of the vehicle, which is detected by an acceleration sensor.
In the technologies of the related art, because momentarily rough terrain (a road with obstacles such as foreign objects on the road such as stones, branches, and pieces of wood, and exposed manholes) on which the vehicle is traveling may have instantaneous changes in road surface μ, values detected by the acceleration sensor and the wheel speed sensor may temporarily largely vary.
For this reason, the estimated road surface slope value may largely deviate from the actual value, and the engine may be prohibited from being automatically stopped even on a road surface with a slope on which automatic stopping of the engine would actually be allowed. Thus, a problem arises in that fuel economy may deteriorate.